Some plankton protists have hard parts, shells or skeletons.

Tintinnids are ciliates which make a lorica (or shell) into which the ciliate cell can contract. Radiolarians make skeleletons of silica or stronium. Silicoflagellates make a skelton out of silica. In the image, only the skeleton of the silicoflagellate is visible. The image is from a sample taken at 90 m depth in Oct 2004 from a station midway between Nice, France and Corsica in the NW Med. The radiolarian is Cornutella profunda, the tintinnid is Dicytocysta mitra and the silicoflagellate is Dictyocha speculum